The present invention relates to an apparatus for displaying half tone images on a bilevel display.
There are two conventional methods for representing half tone images with bilevel values:
(1) the dither method and (2) the density pattern method. In the dither method, one display pixel is associated with one pixel of a continuous tone (multi-level) image which is an original image, and continuous tone values are quantized by using threshold values defined beforehand with respect to respective pixel positions, each pixel thus being determined to be either black or white. In the density pattern method, one pixel of an original image is associated with an nxn dot matrix imitatively representing the continuous tone value of that pixel to display bilevel images. Descriptions in COMPUTER GRAPHICS AND IMAGE PROCESSING 5, 1976, pp. 13 to 40 and JP-A-54-144141 relate to the former cited method. On the other hand, JP-A-61-281370 and JP-A-56-66970 relate to the latter cited method.
The present invention relates to the former method. The conventional techniques will now be further described by referring to FIGS. 2, 3 and 4. Assuming now that a multi-level image 1 is represented by 5 levels, a 2.times.2 matrix (dither matrix) 7 representing 5 grades as shown in FIG. 4, for example, is produced. A 2.times.2 pixel region 3 of the multi-level image 1 is associated with the dither matrix as shown in FIG. 2. Level values of pixels forming respective elements of the region 3 are compared with quantization thresholds forming corresponding elements of the dither matrix. When the level value of a pixel of the original image is larger than the threshold value, a pixel of a displayed bilevel image 2 (such as a pixel 5 corresponding to a pixel 4 of the original image 1) is displayed as a white pixel. Otherwise, a black pixel is displayed. This processing is generalized in FIG. 3.
In FIG. 3, n denotes size of the dither matrix, x a coordinate value of the original image in the x direction, y a coordinate value of the original image in the y direction, i a row number of the dither matrix, j a column number of the dither matrix, I(x, y) a tone value of a pixel (x, y) of the original image, D(i, j) a value of an element (i, j) of the nxn dither matrix, and B(x, y) a displayed value of a pixel (x, y) of the bilevel image. Further, dither matrices 8 and 9 of FIG. 4 exemplify threshold setting proposed in the prior art for n=3 and n=4, respectively. Further, it is assumed that luminance becomes higher as the tone value becomes larger.
It is assumed that in a work station a multi-level image, particularly a document generated by software, which has been developed for a multi-level display such as a color display or a half tone display, is displayed on a bilevel display by using the prior art. If the contrast with respect to the background color is small in this case, the image becomes extremely hard to see. Further, a drop in display resolution caused by use of the dither method prevents making the contrast small as compared with color. The prior art thus has problems associated therewith.
The establishment of the dither matrix of the prior art has a problem in that flicker is caused when a display of an interlace scheme is used. Further, when comments are added to a bilevel image, the display, of the bilevel image to be operated with half tone and the emphasis thereof has not been considered.
In addition, a problem as shown in FIG. 5 is caused as described below when a half tone display device using the conventional dither method is applied to a multiwindow system. It is assumed that a window 201 on a display screen 200 is moved to the position of a window 202, a drawing area 204 is newly added, and half tone display using the dither method is applied thereto. Originally, the half tone should be displayed on the drawing area 204 and should have the same phase as that of a drawing area 203. Since the method shown in FIG. 3 is used in the prior art, however, the phase of dither is different from that of the drawing area 203 as illustrated in a drawing area 206 within a display screen 205, resulting in a problem that the user feels uncomfortable.